1 A woman walks past the Google offices in Dublin. On Wednesday the internet giant announced the end of targeting ads based on individual users' web-browsing behavior. Alphabet Inc.'s Google has crushed almost all its competitors in the world of digital-advertising technology. But one rival is emerging as the best hope to challenge the tech giant--if it manages to keep up its momentum. The Trade Desk Inc., which specializes in helping companies buy online ads across publishers' websites, has done what others failed at: eating into Google's share of the market. While Google dominates that area of ad-buying with about 40% of the business, Trade Desk is up to nearly 8% and its share is growing faster than Google's, according to ad-tech consultancy Jounce Media.